The present invention relates generally to the recording of video signals, and more particularly to the conversion and recording of digitized baseband component video signals such as red, green, and blue (RGB) or any of its linear transformations (e.g., YUV or YIQ).
Specialized combinations of computer hardware and software, such as QuickTime.RTM. by Apple Computer, Inc., allow users to create and edit video movies using a combination of video, graphics, and sound data. Each frame of a movie exists in a digitized baseband component format, which allows the images to be stored and manipulated by a computer. While the digitized format is ideal for creating and editing movie frames on a computer, this format is not compatible with video cassette recorders (VCRs) which require a composite television signal (usually NTSC or PAL) input.
As is well known to those skilled in the art, NTSC and PAL composite television signals are recorded using frequency modulation of the luminance (Y) component, which is then used as the AC bias signal for the down converted QAM encoded chrominance components. Quadrature amplitude modulation encoding, or QAM, is the suppressed carrier amplitude modulation of two subcarriers in quadrature (I and Q for NTSC, or U and V for PAL). The signal parameters of these and other composite television standards can be found in "Report 624-4, Characteristics of Television Systems," Reports of the CCIR, 1990, pp. 1-33.
Prior practice for converting and recording digitized component video signals onto analog video tape has been to convert the digitized components into RGB analog components, encode them to form a composite television signal, and then apply this composite signal as input to a video cassette recorder (VCR). Common consumer VCR standards are VHS, S-VHS, Video-8, and Hi8. The parameters of VHS are defined in Helical-scan Video Tape Cassette System Using 12.65 mm (0.5 in) Magnetic Tape on Type VHS. IEC Standard 774, First Edition, 1983. The parameters of S-VHS (a proprietary system of JVC) are defined in a paper by Y. Nagaoka, M. Tsurata, and H. Fujiwara entitled "High Performance VTR Based on the S-VHS Format," IEEE Trans. Consumer Electronics, 34(3), August 1988, pp. 560-565. The parameters of Video-8 are defined in Helical-scan Video tape Cassette System Using 8 mm Magnetic Tape--Video 8, IEC Standard 843, first edition, 1987. The parameters of Hi8 (a proprietary system of Sony Corporation) are defined in a paper by K. Tsuneki, T. Ezaki, and Y. Kubota entitled "Development of the High-Band 8 mm Video System", IEEE Trans. Consumer Electronics, 35(3), Aug. 1989, pp. 436-441.
All four VCR systems described above require a composite television signal input which is processed for recording by separating the luminance and encoded chrominance components. These signals are then further encoded for video recording. The luminance signal is applied to an FM modulator, and the encoded chrominance is down-converted to a lower subcarrier frequency. All of the video recording systems use a frequency division multiplex approach where encoded luminance and chrominance occupy separate well defined bands, with strict requirements as to modulated signal bandwidth.
Artifacts commonly associated with composite television signal encoding are cross-chroma and cross-luma (together generically called cross-color) and smear. Cross-color is caused by crosstalk between luminance and chrominance signals. This problem is often quite severe on synthetic imagery, such as graphics. Smear is caused by excessive delay between different channels, and is corrected through the use of matched analog filters and delay lines. Each stage of the process described above (analog component generation, composite television signal encoding, and video recording) is typically accomplished with analog circuitry. This analog signal processing adds unavoidable noise and distortion to the recorded video signal.
Accordingly, an object of the present invention is to convert and record digitized component video signals onto analog video tape without first generating the associated composite television signals.
Another object of the present invention is to convert and record digitized component video signals onto analog video tape with fewer processing stages to suppress noise and distortion.
Still another object of the present invention is to convert and record digitized component video signals onto analog video tape using digital processing up to the final analog video recording stage to further suppress or eliminate noise and distortion.
Yet another object of the present invention is to convert and record digitized component video signals onto analog video tape avoiding cross-color artifacts associated with composite television signal encoding.
Still another object of the present invention is to convert and record digitized component video signals onto analog video tape while reducing analog related "smear" by digital processing.
Yet another object of the present invention is to convert and record digitized component video signals onto analog video tape while eliminating the drift, alignment, and calibration problems of a conventional analog implementation.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.